The major purposes of this research are: (1) to explore the extent, the nature, and the clinical importance of heterogeneity of erythrocyte NADH methemoglobin reductase, and (2) to more fully analyze the methemoglobin-reducing process in human erythrocytes and the physiology of the NADH methemoglobin reductase system. The methods employed include electrophoresis in starch and polyacrylamide gels; chromatographic separations using Sephadex, calcium phosphate gels, and a variety of ion-exchange resins; spectrophotometric assay of enzymes; fractionation of erythrocytes according to their density; immunization of rabbits with purified enzymes; and the detection of raised antibodies utilizing immunodiffusion and neutralization techniques.